1. Field of the Invention
The invention relates to the field of high temperature pyrometry. It proceeds from a high temperature pyrometer for temperature measurement in gas turbines, from a gas turbine with a high temperature pyrometer, and from a method for monitoring a gas turbine with the aid of a high temperature pyrometer.
2. Discussion of Background
Different methods of temperature measurement in gas turbines are known from the prior art. The fundamental physical measurement principles are based, inter alia, on the temperature dependence of an electric resistance, the Seebeck effect (thermocouple), a color reaction (thermocolor), the sound velocity in gases or the spectral distribution of scattered or emitted electromagnetic heat radiation.
Temperature sensors for gas turbines have to withstand extreme loads in respect of temperature, pressure and vibration. Conventional thermocouples age very quickly under these conditions of use. Moreover, rotating parts are also to be measured, this being possible only by means of complicated telemetry. The use of thermocolors is restricted to experimental investigations. Active laser measurement methods, such as, for example, Rayleigh scattering or CARS (Coherent Antistokes Raman Scattering), although being contactless, are complicated and difficult to implement.
It was therefore soon recognized that radiation pyrometry, as a passive optic method, is more suitable for temperature measurement in gas turbines under operating conditions. The Patent Application WO 86/00131 discloses an optic pyrometer with a viewing channel from outside the turbine as far as the first row of moving blades. The measuring apparatus is distinguished in that the pyrometer and all the optic components, including detectors, are separated from the high temperature and high pressure environment of the gas turbine by a viewing window. Viewing channels are considered to be all those connecting paths which extend in the flow duct essentially in a straight line from the first row of moving blades through a spacing between two guide blades toward the turbine casing on the combustion chamber side and which do not touch any turbine parts carrying hot gas. The viewing channel is designed to be wide enough to maintain free viewing communication, even in a thermally deformed machine, and, moreover, for different zones of the moving blades to be imaged by adjustment of the beam path within the channel. The viewing channel and viewing window are scavenged by compressed air. In a measuring setup of this type, the temperatures of guide blades, of the hot gas duct, of the burner walls or of the gas can also be measured. The main disadvantage of this pyrometer is the inherent limitation of measuring accuracy due to the enormous measuring distance of up to several meters. This is because, as a result, the area and position of the heat image to be detected can only be inadequately checked.
A related high temperature pyrometer with a viewing channel and a pressuretight viewing window is provided by Land Instruments International, Inc. under the name TBTMS (Turbine Blade Temperature Measurement System). According to FIG. 1, the viewing channel 2 terminates, outside the turbine casing 10, with a viewing window 3. There, the heat radiation is fed by optics 4 into a fiber 5 and is led by this out of the soundproofing hood (enclosure) 11. In a detector 6, the optic signal is converted into an electric signal which is supplied to measuring electronics 8 via a signal line 7. The infeed optics 4 and the connecting fiber 5 are exposed to ambient temperatures of at most 400.degree. C. The increased risk of contamination of the optics 4 in the vicinity of the gas turbine interior has to be countered by vigorous air scavenging 9. For this purpose, the viewing channel 2 inside the turbine may have a protective tube of a length of up to 1.2 m, which consists, for example, of silicon carbide (SiC) and which withstands temperatures of up to 1550.degree. C. and keeps away soot particles, etc.
A great disadvantage of the Land pyrometer 1 is the complexity of the optic sensor head to be mounted on the turbine casing 10. This is because said sensor head comprises an adjusting head for the optics 4 for imaging the radiation object, the pressure and temperature partitioning, including the viewing window 3, complicated air or water cooling means and the protective or viewing tube together with the scavenging system. The total weight of a sensor head of this type may greatly exceed 50 kg. Also, the viewing tube diameter of approximately 15 mm-60 mm greatly restricts the possible installation locations in gas turbines and may cause undesirable mechanical weakening of turbine parts. As will be explained in more detail later, the fact that the viewing channel 2 is linear means that this restriction leads to very flat observation angles at which the moving blades can be imaged, said angles falsifying the temperature measurement. Furthermore, at more than 1 m, the measuring distance is once again very large. The measuring spot therefore covers too large a zone of the moving blades and, under some circumstances, also part of the rotor, with the result that further measuring errors occur. Evidence of temperature errors of more than 50.degree. C. has been found in tests with the Land pyrometer.
It is state of the art, furthermore, to equip a high temperature pyrometer with a solid optic sensor head in the form of a rigid lightguiding sapphire rod resistant to high temperatures. FIG. 2 shows a version from the Luxtron Corporation, Accufiber Division. A black cavity emitter 13 on the tip of the sapphire rod 14 serves as a measuring probe, which is held in the hot gas stream and is heated there. Once again, the heat radiation is fed outside the hot gas zone, that is to say outside the turbine casing, via an optic coupler 15, into a low temperature fiber 16, is led out from the soundproofing hood and is supplied to a detector 17 having an optic filter 18 and photodiode 19. In order to measure moving blade temperatures, the sapphire tip may also be polished flat and bent, in order to absorb the heat radiation from the desired observation object contactlessly.
The sapphire rods typically have lengths of up to 0.4 m and diameters of more than 1 mm. They become extremely hot toward the tip. Measurement values are falsified mainly due to characteristic radiation, individual absorption, radiation losses to the cooler environment and the lateral infeed of heat radiation along the freestanding sapphire rod. In the case of the closed embodiment with a cavity 13, measuring errors due to heat conduction in the sapphire rod additionally occur. In unfavorable instances, the measured temperature no longer has much in common with the hot gas or object temperature. Furthermore, in both embodiments, the flow deformation of the rods 14 above 1300.degree. C. presents problems, and because of this flow deformation the sapphire length which can be exposed to the gas flow is limited to less than 10 mm. Watercooled carrier probes and sapphire supporting tubes are used for protection. These solutions are unsatisfactory, however, since the carrier probe disturbs the gas stream and the supporting tube is exposed to pronounced temperature gradients between the windward and the leeward sides in the gas stream and consequently to high internal stresses.
Known pyrometric signal evaluation methods, such as are specified, for example, in the manual "Temperaturstrahlung" [Thermal radiation] by W. Pepperhoff, Verlag von Dr. D. Steinkopff, Darmstadt 1956, are used to calculate the temperature from the heat radiation. In particular, the spectrum of heat radiation for determining temperatures may be evaluated in a monochromatic, bichromatic or wideband manner. Bichromatic pyrometry is useful, above all, for eliminating the influence of variable emissivity of the radiation object.
The pyrometric sensors mentioned may, in principle, be employed to determine a mean temperature of a row of moving blades or individual temperatures of the individual moving blades. The mean temperature is a useful parameter for protecting the gas turbine, for example for limiting the thermal load on the gas turbine by automatic load reduction. The individual temperatures are suitable as an early warning of overheating of the moving blades, for example on account of blocked or damaged cooling ducts.
However, in order to perform these functions, extremely stringent requirements have to be placed on the accuracy and longterm reliability of the temperature measurement. The pyrometers available at the present time do not achieve the desired measuring accuracy for the reasons mentioned. Moreover, they have a complicated design and are bulky, they require highpowered cooling systems and, overall, are difficult to integrate into gas turbines. The advantages of flexible optic transmission are fully utilized only at low temperatures. By contrast, at high temperatures, the geometry of the viewing channel or sapphire rod restricts the flexibility of the pyrometers and their adaptability to the demanding gas turbine environment.